Tropospheric ozone climatology over Irene, South Africa, from 1990 to 1994 and 1998 to 2002

Ozonesonde measurements over Irene in South Africa are reported for the period 1990 to 1994 and a more recent period, 1998 to 2002, when the station became part of the Southern Hemisphere Additional Ozonesondes (SHADOZ) network. Irene displays the characteristic Southern Hemisphere springtime tropospheric ozone maximum, but its seasonal features are modulated by both tropical and midlatitude influences because of its location (25°54′S, 28°13′E) on the boundary of zonally defined meteorological regimes. The tropical savanna biomass burning signature, namely, the spring maximum, is less distinct in the lower troposphere than at stations closer to biomass burning source regions nearer the equator, although long‐range transport and recirculation in the subtropical anticyclonic gyre over southern Africa permit the buildup of relatively high springtime midtropospheric ozone. Midlatitude dynamical influences are evident, predominantly in winter when upper tropospheric ozone is enhanced as a result of stratospheric‐tropospheric injection of ozone. Mean tropospheric ozone values range between 40 and 60 ppbv throughout the year and increase by ∼20 ppbv in spring. The increase (∼10 ppbv) in surface and lower tropospheric ozone between the two time periods is attributed to an increase in urban‐industrial emissions. A classification of ozone profiles using a cluster analysis has enabled the delineation of a background and “most polluted” profile. Enhancements of at least 30% occur throughout the troposphere in spring, and in certain layers, increases close to 100% are observed.